Techniques for COFDM DTV broadcasting are prescribed in the ETSI TS 302 755 V1.3.2 Technical Specification titled “Digital Video Broadcasting (DVB); Frame structure channel coding and modulation for a second generation digital terrestrial television broadcasting system (DVB-T2)” published in April 2012 by the European Telecommunications Standards Institute. This DVB-T2 standard is based on time-division multiplex of T2 frames of COFDM symbols possibly having future-extension frames (FEFs) interleaved therewith, every six consecutive ones of which frames is considered to constitute a “super frame”. The duration of a T2 frame can be of as long a length as 250 ms (milliseconds) and begins with a P1 portion of a preamble, which P1 portion signals which of various ways for transmitting COFDM DTV signals is currently in use. The number of ways for transmitting COFDM DTV signals that can be signaled in each P1 preamble is 24, 8 in an S1 period and 16 in an ensuing S2 period. The three-bit S1 field indicates whether the currently incoming transmissions are T2 frames using 64,800-bit LDPC codeblocks, T2-lite frames using 16,200-bit FEC codeblocks, or FEFs using some still-to-be-decided form of FEC coding. The three-bit S1 field further indicates whether the P2 portion of the preamble following the P1 portion is to be interpreted presuming single-input/single-output (SISO) reception or multiple-input/single-output (MISO) reception. The first 3 bits of the 4-bit S2 field are referred to as S2 field 1. When the preamble format is of the typeT2_SISO, T2_MISO, T2-LITE_SISO or T2-LITE_MISO, S2 field 1 indicates the FFT size and gives partial information about the guard interval for the remaining symbols in the T2-frame. The ensuing single-bit S2 field 2 that concludes the S2 field indicates whether the preambles of all the T2 frames in a super frame are all of the same type or not.
DVB-T2 permits the time-division multiplexing of several physical layer pipes (PLPs), and information concerning the operating characteristics of those PLPs) is conveyed by first-layer (L1) signals that immediately follow the P2 portion of the preamble. Each L1 signal consists of an L1-pre signaling initial portion followed by an L1-post signaling portion. All L1 signaling data, except for the dynamic L1-post signaling, shall remain unchanged for the entire duration of one super-frame. Therefore, any changes implemented to the current configuration (i.e., the contents of the L1-pre signaling or the configurable part of the L1-post signaling) shall always occur at the transition from one super-frame to the consecutive next super-frame. The L1-pre signaling is 200 bits in size. It specifies whether the L1 signal is a repeat or not, the length of guard intervals, the type of peak-to-average-power ratio (PAPR) reduction employed, the natures of FEC coding and the modulation of OFDM carriers in the ensuing L1-post signaling, the size of the ensuing L1-post signaling, pilot carrier pattern, whether L1-post signaling is scrambled or not, and other items related to super-frame structure. The L1-pre signaling concludes with 32 parity bits of cyclic redundancy check (CRC) coding. The so-called “configurable” portion of L1-post signaling is 257 bits in size and comprises information relating to FEFs and to PLPs. The “configurable” field can be followed by “dynamic” and “extension” fields. The L1-post signal concludes with 32 parity bits of CRC coding followed by bits for padding the L1-post signal to prescribed size.
The complete L1 signal includes 457 bits for signaling. Nonetheless, additional signaling capability may be required, and per custom this has been afforded by so-called “in-band” signaling wherein control signals replace portions of the broadcast normally allocated to DTV signal. In-band signaling undesirably complicates time-division multiplexing of components of the baseband signal to be up-converted in frequency and then broadcast. More important, in-band signaling undesirably complicates de-multiplexing of components of the baseband signal in a COFDM DTV receiver. In-band signaling tends to reduce digital payload in the DTV broadcast system, but this undesirable effect can be mitigated by replacing the bits for padding the L1-post signal with at least part of the in-band signaling.
The Advanced Television Systems Committee (ATSC) is an international consortium of television broadcasters, manufacturers of equipment for transmitting DTV signals, and manufacturers of equipment for receiving DTV signals. ATSC seeks to develop a “universal” standard for terrestrial over-the-air COFDM DTV broadcasting in most countries in the world, which countries prescribe a variety of permissible radio-frequency channels for such broadcasting. These RF channels have different bandwidths in different countries, six megahertz (MHz) bandwidth being used in North American countries and 8 MHz bandwidth being used in European and Asian countries. Manufacturers of equipment for receiving DTV signals are apt eventually to desire development of a bandwidth-agnostic way to specify the nature of RF channels selected for reception. A bandwidth-agnostic a way of specifying the nature of RF channels selected for reception would allow portable DTV receivers to continue to be able to receive DTV signals usefully when transported from a country in which 6 MHz RF bandwidths are used in over-the-air DTV broadcasting to a country in which 8 MHz RF bandwidths are used for over-the-air DTV broadcasting, or vice versa. A bandwidth-agnostic way of specifying the nature of RF channels selected for reception might also reduce the number of different electronics designs for DTV receivers that a DTV receiver manufacturer might have to produce to encompass a global market for them.
The Long Term Evolution (LTE) cell-telephone standard specifies coded orthogonal frequency multiplexed (COFDM) carriers being used to convey down-link telephonic signals and single-carrier frequency-division multiple-access (SC-FDMA) being used to convey up-link telephonic signals. SC-FDMA is favored over OFDM in the uplink communications where lower peak-to-average power ratio (PAPR) greatly benefits the mobile terminal in terms of transmit power efficiency and reduced cost of the power amplifier. The signaling as to which of various ways for transmitting COFDM down-link telephonic signals is currently being received by a cell telephone is specified by the modulations of 72 COFDM carriers located in the central portion of the radio-frequency (RF) channel, 12-carrier-wide groups of which 72 carriers each include receiver synchronization signaling and 84-bit Master Information Blocks (MIBs). The 72 COFDM carriers use Evolved Universal Terrestrial Radio Access (E-UTRA) modulation specified in the 3GPP TS 36.211 V9.1.0 standard published in March 2010 by the 3rd Generation Partnership Project (3GPP). Current practice is further described in the ETSI TS 125 213 V10.0.0 Technical Specification titled “Universal Mobile Telecommunications System (UMTS); Spreading and modulation (FDD)” published in May 2011 by the European Telecommunications Standards Institute and corresponding to the 3GPP TS 25.213 V 10.0.0 standard.
The location of E-UTRA modulation of 72 COFDM carriers in the middle of the RF channel is bandwidth-agnostic, the information as to RF channel bandwidth being conveyed within the MIB block. Also, the E-UTRA modulation conveys a considerable number of bits of information in patterns of pilot carriers therein. These observations inspired the inventors to consider to whether or not the E-UTRA modulation might be adapted to meeting goals they perceived to exist in prior-art COFDM DTV broadcasting systems.
In a first type of E-UTRA modulation used for down-link in cell telephony, the 72 COFDM carriers are considered to be grouped in six groups of 12 adjacent carriers within frames 10 milliseconds in duration. Each frame is apportioned into ten sub-frames of like duration, and each sub-frame consists of seven sample periods. Each sub-frame includes seventy-two subcarriers times seven sampling periods for a total capacity of 72×7=504 bits. Every 5 milliseconds 63 central ones of the 72 COFDM carriers in this E-UTRA modulation are briefly modulated for one sample period by respective elements of a Zadoff-Chu sequence, also known as a generalized “chirp” sequence. In the next sampling period 62 central ones of the 72 COFDM carriers are modulated by respective elements of two concatenated 31-bit pseudo-random noise (PN31) sequences as additively scrambled in accordance with the preceding Zadoff-Chu sequence. In the context of the first type of E-UTRA modulation, the Zadoff-Chu sequence is referred to as “primary synchronization signal”, and the scrambled concatenated pair of PN31 sequences is referred to as “secondary synchronization signal”. Pilot carriers at positions other than those in these synchronization signals are referred to as “reference symbols” and are used to specify sectors and cells in cell telephony.
COFDM DTV broadcasting systems use a large number of COFDM carriers. Typically, this number approaches 2 048, 4 096, 8 192, 16 384 or 32 768. In DVB-T2 these numbers are 1 705, 3 409, 6 817, 13 633 and 27 265 respectively. Generally, these numbers are loosely referred to as 2K, 4K, 8K, 16K and 32K, respectively. The inventors observed it to be desirable that as many as possible of the COFDM carriers be dedicated to conveying DTV signals, rather than metadata descriptive of the DTV signals. Accordingly, the inventors prefer modifying the first type of E-UTRA modulation used for down-link in cell telephony so as to include just 64 COFDM carriers when such modulation is instead used to transmit metadata concerning DTV signals in a DTV broadcasting system. Preferably such modification includes grouping the 64 COFDM carriers in eight groups of eight adjacent carriers and extending sub-frames of metadata to include eight sample periods. Then, each sub-frame of metadata includes sixty-four subcarriers times eight sampling periods for a total capacity of 64×8=512 bits.
Sampling periods are adjusted to be the same rate as those used in the DTV signal and are several times longer than for the E-UTRA modulation used for down-link in cell telephony. Frame size for the metadata is likely to vary depending upon FFT size and upon the durations of T2 frames and FEFs. A sophisticated practice is to time accurately the primary synchronization signals in the E-UTRA modulation used in conveying metadata about DTV respective to the occurrences of T2 frames and FEFs in the COFDM DTV signal.
In the continuing development of DTV broadcasting, new systems are apt to be developed that cannot be usefully received by DTV receivers already in the field. It would be useful if receivers could be signaled as to the general type of DTV broadcasting system or other broadcasting system currently occupied an RF channel. This would facilitate receivers determining whether a currently received RF broadcast signal was of a general type that the receiver was capable of usefully receiving. This feature is especially useful for receivers designed for receiving signals broadcast according to a variety of transmission standards used in different countries.
The “universal” standard to be developed for terrestrial over-the-air COFDM DTV broadcasting is expected to be used for many years and is referred to as ATSC 3.0. However, a concern of the broadcasting community as expressed in ATSC is that the development of future standards for terrestrial over-the-air COFDM DTV broadcasting be accommodated by transmitters specifically signaling receivers as to whether they transmit COFDM TV according to DVB-T2, ATSC 3.0, some later version of ATSC 3.0 or some future broadcasting standard. Increasing the number of bits in the “configurable” portion of L1-post signaling to accommodate additional parameters descriptive of some later version of ATSC 3.0 or some future broadcast standard undesirably increases signaling overhead.
An alternative approach is to encode a digital signature sequence that is the key for specific interpretation of further bits of signaling in an L1 signal of a data frame similar to a DVB-T2 data frame. This facilitates one of a plurality of different broadcast standards using some or all of the further bits in an L1 signal differently from another or others of that plurality of different broadcast standards. It is desirable that a number of digital signature sequences can each be readily distinguished from each and all of the others, but this tends to require digital signature sequences quite a few bits long, which also tends to increase signaling overhead, although perhaps to lesser degree than extending the L1 signal or augmenting the L1 signal with in-band signaling that replaces data.